Telephone trunking system.



PATENTED MAR. 19; 1907.

F. w. DUNBAR.

TELEPHONE TRUNKING SYSTEM.

APPLICATION FILED AUG. 25, 1902.

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APPLICATION FILED AUG. 25, 1902.

THE NORRIS PETERS co, WASHINGTON, n. c e

PATBNTED MAR. 19; 1907.

1?. W'. DUNBAR. TELEPHONE TRUNKING SYSTEM.

APPLICATION FILED AUG. 25, 1902.

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UNITED STATES FRANCIS W. DUN BAR, OF CHICAGO PATENT OFFICE.

, ILLINOIS, ASSIGNOR TO KELLOGG CORPORATION OF ILLINOIS.

TELEPHONE TRUNKING SYSTEM.

Specification of Letters Patent.

Patented March 19, 1907.

Application filed August 25,1902. Serial No. 120,878.

To all whom it may concern.-

Be it known that I, FRANCIS W. DUNBAR, a citizen of the United States ofAmerica, and a resident of Chicago, county of Cook, and State ofIllinois, have invented a certain new and useful Improvement inTelephone Trunking Systems, of which the following is a specification.

My invention relates to improvements in telephone trunking systems andapparatus whereby telephone subscribers whose lines terminate atdifierent central ofiices or central stations may be connected togetherfor conversation.

The trunking system of the present invention extends between oflices atwhich subscribers lines of diiferenttypes terminate. At the outgoingoffice the subscribers lines are provided with cut-oii relays and withtest-terminals connected in local circuit, and the subscribers bells aregrounded from one side of the line. The operators cord-circuits areadapted to cooperate with/the said lines and are conductively dividedbut inductively united through the intermediation of a repeating-coil.At an intermediate point of the repeating-coil-winding bridge asupervisory relay is leg ed to ground through ith this arrangement ofcord-circuits when a connection is established and the subscriberstelephone is upon its hook a path for current from the central batteryis provided through the supervisory relay and over the line to thesubscribers station; but when a subscribers receiver is in use the'pathfor said current is disrupted and the supervisory relay is inoperative.At the incoming office the subscribers lines are provided with cut-oiirelays actuated over a portion of the talking-circuits, and thetestingcontacts of the jacks are connected directly with one side of thetalkingcircuits. This trunk-circuit is adapted for use with. the saidsubscribers lines at the incoming office and with the special type ofcord-circuit above described at the outgoing o'ffice,the connectionbeing so arranged that the usual method of operation of trunk-cir cuitsis obtained.

My invention is shown in the accompanyin drawings, in which igure 1 is adiagram of a system of the kind above described. Fig. 2 is a diagram ofa modification in which the repeatin -coil at the incoming end of thetrunk is rep aced by condensers. Fig. 3 is a diagram of still anothermodification in which condensers are employed and in which thetrunk-relay is actuated by current over both sides of the trunk, andFig. 4 is a diagram of a different form of the invention in whichcondensers are employed at the incoming end and in which the trunk-relayis actuated by current over the opposite side of the line from thatdisclosed in Fig. 1.

Referring to Fig. 1, L designates one of the plurality of subscriberslines terminating at the central oflice C. This line extends in twolimbs 2 and 3 from the subscribers station A to the said central ofiiceC, where it is fitted with suitable answering and multiple jacks J and JAt the subscribers station the usual telephone instruments are providedand consist of a transmitter 4, a receiver 5, a hell 6, and a localbattery 7. The switch-hook 8 is adapted in its normal position and whenthe receiver is placed thereon to open the circuit between the limbs ofthe line as well as the local circuit of the transmitter and to closethe tip-limb 2 through the grounded signaling-bell 6. In its uppermostmitter 4, local battery 7, and primary ofthe substation induction-coilis closed, and at the same time the main line is closed through thereceiver and secondary of said coil. The cut-ofi relay R is energized bycurrent from the battery B over a set of conduction including wire 9 tojunction-point 10, thence through the cut-off relayR and by theswitchboard conductor 11, the auxiliary springs y' of the jacks, andthrough the cooperating earth-contacts 7'. These. contacts f and j" areclosed together when a plug is inserted in a ack and the cut-ofi relayis actuated. The test-rings i of the jacks are connected toether bymeans of conductor 12 with the 'orward contact of spring 13 of thecut-off re lay R, the latter being grounded through a resistance-coil 14of suitable dimensions. The contacts 7' and 7' of the jacks are normallydisconnected from the line, but are connected by Wires 15 and 16 withthe forposition a local circuit containing the trans-V ward contacts ofsprings 17 and 18 of the cut-- oil relay, whereby when the cut-oil relayis i ringing the desired subscriber, and with sigaetuated these contactsare connected with the limbs of the telephone line. The spring 17 of thecut-oil relay is normally grounded, while spring 18 is normallyconnected by means of conductor 19 with the line-signal S, the otherterminal of which is joined by conductor 9 with the live pole of batteryB, whereby when the subscriber takes up his receiver in calling thecentral oilice a complete path for current from said battery is providedover the metallic line and the line-signal S to actuate it, and therebyindicate a call.

The cord-circuit is provided with an answeringplug P and with acalling-plug P each of which has a tipeeontact p and a sleeve or stemcontact 19 These contacts-of -lthe answering-plug P are looped togetherby means of the tip-strand t and sleeve-strand s and the helices 0 and 0of the repeatingcoil 0. The plug P is likewise connected in the loop bythe tip-strand t the sleevestrand s and the helices 0 and 0 of the saidrepeating-coil. A supervisory relay 7 is joined by conductor 20 with thejunction point 21 of the loop of the plug P, and its other terminal isconnected by conductors 22 and 23 with the live pole of the battery B. Asupervisory lamp S is included in a circuit with the battery B by meansof the ar mature and forward contact of said supervisory relay r. Theopposite plug it is associated therewith, a supervisory relay rcontrolling supervisory signal S in a manner similar to the lower partsconnected with the other plug of the cord-circuit. The operatorstelephone sets comprise the receiver 24, adapted to be bridged acrossthe cord-circuit by the listening-key 7c is, said bridge including thesecondary 25 of her induction-coil and the condenser 0. Her transmitter26, together with the primary 27 of her inductioncoil, may be chargedfrom the battery B. The conductors 28 and 29 serve to connect theoperators instruments with the norm ally open contacts of key k by meansof which she is able to connect her telephone with the order circuit orwire 33, leading to the other exchange. The calling-generator g isadapted to be connected, together with the resistancelamp 30 betweenground and the tip-strand of the calling-plug, by means of thetip-contact of key is, the sleeve-contact of key k serving to ground thecorresponding strand to provide a path for the ringing-current in casethe subscriber answers during the process of ringing.

A trunk-circuit is shown extending between the central oi'iiees C and Csaid trunkcircuit being fitted at the outgoing end with jacks J 3 and Jand extending to the central oilice C in two limbs and 36, where thetrunk is provided with suitable means for testing the condition of thewanted line, for

nals to enable the operator to properly attend the connection. The tipand sleeve trunk conductors 35 and 36 terminate at the incoming oiiicein windings w and U72 oi the repeating-coil and from a point betweensaid windings a conductor 38 extends to the normal contact of spring 37of t-hetip-rclay r. The said spring is C-OTU'lOOiOd, by means of aconductor 39, with the trunk-relay W, the opposite terminal of which isgrounded. The opposite windings w and W oi? the repeating-coil N, withan interposed condenser are connected between the tipstrand t and thesleeve-strand s ot the ilexible end of the trunk-circuit. These strandsterminate in the corresponding contacts and p of the trunk-plug P. Thetip-relay r is logged to ground from the tip-strand if of thetrunk-circuit by conductor 36". A sleeve-relay r is connected on oneside with the live pole of the battery B by means oi conductors 4-0 and4-1 and on the other side with the sleeve-strand s. This incoming end ofthe trunk is provided with a special testing apparatus. The 'i orwardportion oi" the tip-strand if is connected with spring -12 oi thesleeve-relay r, while the other portion is connected with the forwardcontact oi said spring. The back contact oi spring 4-2 is joined byconductor 43 with the common high-resistance and high-impedancetestrelay T the opposite terminal oi which is grounded. The armature oithis test-relay serves when actuated to ground one side oi the operatorsinduction-coil 4-4, which winding is connected on the opposite side witha conductor 45, containing the resistance and impedance coil 46 andvjoined to conductor 47, which leads directly to the live pole ot thebattery B. The operators transmitter 48 is in a conductor 49, leadingfrom conductor to ground. A condenser c is connected between conductor49 and the side oi the primary that is adapted to be grounded. Theoperators receiver 50 together with the secondary of her induction-coiland, the condenser e are connected permanently with the order-circuit33, leading to the central oilice C. The conductor it) leads to thepoint. 54, from which point the branch 55, cont aining the ringing-lampS", leads to the back contact of the armature of inching-relay W, saidarmature being connected by conductor to the point 52 upon the conductor52 extending between the spring 57 of tip-rel a r and the front contact01" spring 5] of sleeverelay 1", the latter spring being grounded. asshown. The conductor 56 extends from point 54-. to the front contact oi"spring 57 oi tip-relay r and includes the winding of the locking-outrelay 1'. The forward contact of the armature of this relay connected toconductor 56 at point 59 by means of conductor 58. The back contact ofspring 57 oi tip-relay r is joined by conductor 59 to the back contactof the armature of trunk-relay W, the forward contact of which isconnected by conductor 52 with the back contact of spring 51 ofsleeve-relay r". The armature of the trunk-relay r is connected with aconductor 53, leading to the battery-wire 47 and including the guard anddisconnecting lamp S and the pilot-relay 7", said latter relay servingto control the circuit of pilot-signal S; This incoming end of the trunkis equipped for selectively ringing subscribers upon a party-line. Itcomprises the ringing key contacts k k and k k placed in the strands ofthe trunk-cord and adapted when actuated to suitably connect thealternatingcurrent generator 9 with the plug P This ringing-generatorcomprises the armature 60, the grounded brush 61, the brush 62, fromwhich the alternating current is directed through resistancedamp 63 totip-key spring 14 when depressed, and brush 64 from the splitcommutator-ring, which is adapted to deliver a current of negativepulsations only through lamp 65 to the sleeve-key 71: when the key isoperated. The sleeve contact of key M when operated connects the sleeveof the plug to battery-lead 47 through resistance-coil 66 to maintainthe cut-off relay of the line operated during ringing.

The subscribers line shown in connection with this oflice is of theparty-line type and is connected to subscribers D and D The bell 6 atstation D is connected between the sleeve-conductor 3 and ground, whileat station D the bell is connected between the tipconductor 2 andground. Normally disconnected jacks J J line-relay R cut-off relay R,and the line-signals S are employed. The battery B furnishes current forthe operation of these various relays and signals, as well as fortalking.

In establishing a connection the A subscriber takes up his receiver, andthereby indicates a call through the intermediation of his signal S. Theoperator in response to this call inserts the answering-plug P of hercord-circuit, thereby actuating the cut-off relay to render theline-signal inoperative and to connect the jack-terminals with the limbsof the telephone-line. The supervisory signal S remains inert, for thereason that the path for current from the battery B over thetelephone-line is at this time open.

Upon being informed that a subscriber in the exchange 0 is desired thekey k is depressed to connect her telephone with the operatorsinstrument at the said office, who nominates the trunk to be used.Before connecting the trunk-plug with the wanted line the B operatortests the same with the tip of the incoming trunk-plug. If idle, noclick will be received but if the line is busy the sleeve-contacts ofthe jacks 'of the line will be connected with the live pole of a batterysimilar to the battery B through the sleeve-contact and sleeve-strand ofthe flexible cord of the inserted plug. A complete circuit willtherefore be established from the sleeve-contact of the tested jack,which it is assumed is connected with the live pole of a battery throughthe tip of the plug P tipstrand i spring 42, and back contact ofsleeve-relay r", conductor 43, through the common high-resistance andhigh-impedance test relay or responsive device r to ground. Thistest-relay will therefore 0 erate and ground through its armature anfront contact one side of the primary of the operators induction-coil44, the other side of which is connected by conductor 45 throughresistance 46 and conductor 47 to the live pole of the battery B Thecompletion of the circuit will permit a flow of current therethroughwhich will induce a current in the secondary of the said induction-coiland produce a click in the receiver 50 of the operators set. Theoperator's receiver will respond to this click at all times even thoughthe order-circuit should be open at the exchange, since theelectrostatic capacity of her circuits and that of the order-circuit isample for the purpose of producing the click.

In the three-wire system where the test-circuits are insulated from thesubscriber s lines 9 there is no danger of varying the current flowingin the line to the annoyance of the subscriber during testing but in thetwowire system where the testing-contacts are connected directly withthe talking-circuits some means is necessary or at least desirable toprevent noticeably varying the current upon lines connected forconversation when testing their condition at other sections of theswitchboard. This is accomplished in the circuit described by means ofthe test relay 1", which is of such high resistance as to vary thecurrent upon the tested line but slightly and which is also of highimpedance, so that its current will build up slowly to thereby prevent asudden variation of current on the line. The subscriber or subscriberswhose lines are tested are therefore undisturbed by a sudden or largevariation of current through their instruments. It is apparent that anyother suitable test-responsive device or indicator than the relay 1 canbe employed and that the operators telephone can be used in place of thesame or in the circuit with a retardation-coil, but thearrangement'described is preferred. This relay is preferably common toall the incoming trunks at the one operators position, as indicatedbythe branching lines at 43*. Upon finding the line idle the plug P isinserted in one of the inultiple jacks of the wanted subscribers ine.

The A operator having received the number of the trunk to be usedinserts the callingplug P of her cord-circuit in the jack of the trunkat her section. This act completes a circuit from the live pole of thebattery B over both sides of the trunk and through the branch 38 at theincoming oflice, spring 37 of relay r and conductor 39, through thetrunk-relay r to ground. The supervisory relay r is therefore operatedand causes the signal S to become visible, thus indicating to the Aoperator that the called subscriber has not responded.

The insertion of the incoming trunk-plug into the jack of the wantedline closes a circuit from the live pole of the battery B overconductors 40 and 41, through sleeve-relay r, and thence over thesleeve-strand s of the flexible end of the trunk through thesleevecontacts of the plug P and the jack of the called subscriber, andthence to the cut-off relay R of the Wanted line to ground, thusoperating both the sleeve-relay r and the cut-off relay R, the latterserving to disconnect the line-relay R of the line to render theline-signal inoperative and to connect the normally-disconnected jackswith the limbs of the line extending to the substations. The operationof sleeve-relay r disconnects the test-relay r and closes the tip-strandt through the spring 4:2 and its forward contact, and therebyestablishes the circuit for conversation. It also completes the circuitof the ringing-lamp S from the battery B over conductor 40 to the point54, thence over conductor 55 and through the lamp S to the back contactof the armature of locking-relay r thence through the armature and overconductor 55" to the point 52*, over the conductor 52 to the frontcontact and armature of sleeve-relay r to ground. The completion of thiscircuit causes the operation of the ringing-lamp S and indicates to theoperator that the subscriber has not yet responded. The operation of thetrunk-relay r at this time opens through its armature and back contactthe circuit of the guard and disconnect lamp S, to thereby prevent itsoperation.

Upon the insertion of the trunk-plug P the B operator rings the wantedsubscriber. If the subscriber D is the one desired, the ringing-key 7: cis depressed, which throws ringing-current upon the sleevestrandconnected with the-plug P and operates the subscribers bell over thefollowing path: from ground at the generator brush 61, armature 60 ofthe generator, brush 6%, through resistance-lamp 65, key thence over thesleeve-strand of the cord to the plug P over the sleeve-limb 3 of thetelephone-line to the station D, and thence through condenser 7 and bell6 to ground, the receiver 5 being at this time upon the hook S, andtherefore completing the circuit between the condenser 7 and bell 6.

It will be noticed that just previous to the depression of theringing-key k the cut-off l relay R was receiving a negative currentthrough its coils from the battery B. At the instant the ringing-key 7cis depressed this path of current from the battery B is broken; but anew path is immediately established through the relay R from thenegative source of pulsating ringing-current. The relay R is soconstructed as to be maintained in continuous operation when thisnegative pulsating current is passing through its winding, and aspulsations are used of the same direction as the current furnished bythe battery B the relay R is prevented from even momentarily releasingits armatures at the momentthe ringing-key was depressed.

The hell 6 at the subscribens station may be of the ordinary polarizedtype, as the presence of the condenser 7, which is (a'mneeted directlyin the ringing-circuit, permits the operation of such a bell from. thepulsating source of ringing-current. The tip-spring of ringing-key /r:"is grminded at this time to provide a path to ground for thereturn-eurrmit if the subscriber should respond during ringing. Shouldthe subscriber ,1) be the one desired, the ringing-key 7e" 7c" isdepressed, which serves to connect the generator with the tip-straml ofthe cord-circuit to (merate the bell at the station I). The ringing-cir-:uit may be traced as follows: from ground at the generator brush 61,armature-wimling 60 of the generator, brush (52, through resistance-lamp623, spring 7e" in the tip-strand of the cord-circuit, thence over thetip-strand and tip-comluctor 2 of the line to the station D and thencethrough condenser 7 and nor mally closed contacts of the hook S andthrough bell 6 to ground. The bell is thus operated. To maintain thecut-oll' relay lt operated during ringing, the inner contact of thesleeve-spring tr" is (amneeted, through a resistance-coil (36, with thebatterydraul 47, extending to the live pole of the battery .13. Thisprevents the deenergization of cutoll' relay it while ringing.

It is apparent that the arrangement of the ringing-keys is such thateither key may be used to call a subscriber at whose station theapparatus is connected in the manner shown at A, which shows thearrangement which 1 preferably use where but one subscriber is placedupon the line. in this case the ring ing-circuit would be complete overthe subscribers metallic circuit, the current returning to the groundedside of the ringing-generator by means of the grounded. key ls" orbattery-contact of the ringing-key It".

The subscriber having been c: lled responds by taking up his receiver,and thereby closes a path for current between the limbs of his line fromthe main battery B over conductors 40 and 41, sleeve-strand s of theincoming end of the trunk-line 3 of the telephone line, thence throughretardation-eoil 9 and transmitter 4 at the substation, thence over limb2 of the telephone-line back to the central oflice and over tip-strand tof the incoming end of the trunk, including spring 42 and its forwardcontact of sleeve-relay 7", through conductor 36 and tip-relay r toground. This relay responds to this current and opens through its spring37 and back contact a circuit of the trunk-relay 1", which causes thedeenergization of the supervisory relay 1' in the A cord-circuit and theconsequent retirement of the supervisory signal S thus indieating to theA operator that the called subscriber has responded. Although thetrunkrelay 7" is now deenergized and its armature engages its backcontact, the guard-lamp S is not operated, because its circuit is openedat the back contact and spring 57 of tip-relay The closing of spring 57of the tip-relay r and its forward contact completes the circuit of thelocking-relay r, as follows: from the live pole of the battery B overconductor 40 to the point 54, thence through the locking-relay 1", overconductor 56 to the forward contact and spring 57 of the relay 1',thence over conductor 52 through the forward contact and spring 51 ofsleeve-relay r to ground. The locking-relay 1' is therefore operated andopens through its armature and back contact the circuit of theringinglam S which is extinguished and indicates to t ie operator thatthe called subscriber has responded to his ring. The closing of thearmature of the locking-relay upon its forward contact completes alocking-circuit for said relay from the point 59 through conductor 58,the forward contact and the armature of the relay, conductor 55 to point52 and thence over conductor 52 andthe forward contact and spring 51 ofsleeve-relay r to ground. This circuit and the relay 1* are thereforelocked, and the ringing-lamp S is also locked out during the remainderof the connection and can only be released when the sleeve-relay 1-" isdeenergized, which occurs when the plug P is ulled out.

Should the tip and-sleeve contacts of the plug be crossed when the plugis inserted in a jack and the locking and other relays operated, theywill be immediately released by the opening of the sleeve-strand inringing, and no confusion would result therefrom. The subscribers arenow connected together for conversation, and the supervisory signals atboth exchanges are extinguished.

During conversation the local battery at the substation is furnishingcurrent to the transmitter for conversation, the voicecurrents beingconducted over the metallic line and propagated in the trunk-linethrough the medium of the repeating-coil in the A-cord circuit, and fromthe trunk-line they are repeated by means of the coil W, into the calledline. The battery B at the B office furnishes current over thetrunk-cord and the subscribers line L to the substation upon I said linefor the operation of the transmitter. Thus at one exchange thesubscribers are provided with local-battery transmission and at theother with common-battery transmission.

At the termination of the conversation the subscribers return thereceivers to their hooks, with the result in the case of subscriber Athat the supervisory relay R is operated. by current over one side ofthe line and through the grounded signaling-bell at the substation. Thesupervisory signal S is therefore actuated. When the subscriber D hangsup his telephone, current is cut off from the metallic line and thetip-relay r is denergized, thus closing circuit from the battery B intheA-cord circuit over the trunk-line and permitting the actuation of thesupervisory relay r and the supervisory signal S This indicates to the Aoperator that the called subscriber has replaced his telephone upon itshook. The spring 57 of the tip-relay r at the incoming end of the trunkis now closed to conductor 59; but the trunk-relay 1, having operatedthe circuit of the disconnecting-lamp S is again opened. The A operatorseeing both signals in her cord-circuit exposed Withdraws both plugs,thus restoring all parts at her ofiice to normal position. Thewithdrawal of the plug P at the A exchange deprives the relay 1" at theincoming end of the trunk of operatingcurrent, which is thereforedee'nergized and permits its armature to close upon its back contact thecircuit of the disconnecting-lamp S and the pilot-relay W, said circuitbeing traced from the live pole of the battery B to conductor 47 throughthe winding of the pilotrelay r and through lamp 8, thence overconductor 53, the armature and back contact of trunk-relayW, conductor59, the back contact and armature 57 of tip-relay r thence overconductor 52 to the front contact and armature 51 of the sleeve-relay 1'to ground. The operation of the pilot-relay 7" lights the pilotsignalS", which attracts the operators attention and who upon observing thelighted lamp S is thus informed that the A operator has taken down theconnection at that exchange, and she accordingly withdraws the plug Pfrom the jack of the subscribers line. Upon the withdrawal of thetrunk-plug the sleeve-relay r is denergized and the circuit of thesignal-lamp S as well as that of the pilot-relay 7, opened at the frontcontact and armature 51 of the said relay. Both lights S and S are thusextinguished, and at the same time the circuit of the locking-relay 14is broken at the same point, which relay is therefore def-Energized andpermits its armature to return to normal position. Thedisconnecting-signal S serves also as a guardlamp in case the A operatorinserts the calling-plug P into wrong trunk-jack, for the reason thatthe trunk-relay of that trunk will be operated, while the'isleeve-relayr is not operated. A circuit will therefore be completed from the livepole of the battery B over conductor 47, through pilot-relay T anddisconnecting-lamp S through conduc tor 53, the armature and frontcontact of trunk-relay a, conductor 52 to the back contact ofsleeve-relay 1", thence through the armature 51 to ground. Thedisconnectinglamp S as well as the pilot-lamp S, will therefore belighted and will indicate to the operator that a mistake has been made.At the same time both ringing-lamp S and guardlamp S of the trunk whoseplug is inserted will operate, thus assuring the operator that the trunkhas not been connected with at its opposite end. It will be noticed thatthe sleeve-relay spring 51 controls through its forward contact a groundbranch from point 52 and that from said point two parallel paths arenormally completed to the live pole of the battery, one includingconductor 55 the normal contacts of lockingrelay 7", ringing-lamp Sconductors 55 and 40, while the other includes conductor 52, the normalcontacts of tip-relay r conductor 59, the normal contacts of trunk-relay1, conductor 47 to the battery. hen ground is thrown upon these parallelpaths, as by the closing of the sleeve-relay 1", both lamps areoperated. This can only occur under these circumstances, for the reasonthat in normal working before the subscriber responds both thetrunk-relay and the sleeve-relay are operated, the former serving toopen one parallel path, which permits only the ringing-lamp to operate.

As soon as the subscriber responds the locking-relay operates and holdsopen the parallel path through its contact during the remainder of theconnection. At the same time, although the trunk-relay is no longeroperated, the tip-relay is operated, which again opens the first path,so that lamp S cannot glow. Now when the A operator takes down theconnection lamp S operates; but the other parallel path remains openthrough locking-relay r.

The order-circuit 33, as is indicated by the branch in lines 33,connects the B operator with several A operators either at the same ordifferent exchanges. The A operator is also provided with a plurality ofkeys 7c to connect herself with the different B operators.

Although I have described the method of making connections through themedium of I do not wish to so conhne the ordercircuit, the invention,for it is apparent that the A operator is able by means of her testingapparatus to test the condition of the several trunks before her untilan idle trunk is found, this being accomplished in any well-known mannerand by any well-known apparatus, such as is shown in connection withjacks of line L. Upon so testing and finding an idle trunk she mayinsert the calling-plug P trunk-relay r operates and lights the lamp Swhich lamp has been previously termed the guard-lamp, but in this methodof operation it would act as a calling-signal l'rom the A operator tothe B operator. The B operator upon seeing the signal would dc press herlistening-key is It, which would connect her receiver in series with therepeating-coil windings w and w, these receiver connections beingindicated in dotted lines in Fig. 1. She is therefore able toeomnumicate with the A operator. Upon l arning the desired connectionthe B operator completes the same, as before described. I t will also beunderstood that had the suliiscriber A called for a connection withanother subscriber in the same exchange the line of that subscriberwould have been tested in the ordinary manner by means of the cord-plugl, and the subscriber would have been called by the ringing-generator 1Fig. 2 shows a modification in that the repeating-coil W at the incomingend of the trunk is replaced by condensers c and e, and the trunk-relayr is actuated over thetip side only of the trunk, the connectionsotherwise being the same as in Fig. 1. It is apparent that the partswill operate in the 1 of her cord-circuit, with the result that. the l lsame way and the same results will be accomplished as in the case ofFig. 'l. l t is therefore deemed unnecessary to describe the operationor parts in, detail.

Fig. 3 represents the circuit in which eondensers c and c are employedin place of the repeating-coil windings and in which the trunk-relay ris actuated by current over both sides of the trunk and ground insteadof merely over the tip side ot the trunk and ground, as shown in Fig. 2.Otherwise the l construction and arrangement is the same as in the saidFig. 2, and the operation is the same as in both Figs. 1 and 2 and doesnot need further description.

Fig. l shows the condensers employed as in the two preceding ligures;but the trunk-relay is actuated by current over the sleeve side of thetrunk instead of over the tip side, as in Fig. 2. The complete (merationof this system also will be understood from what has been said inreference to Fig. 1.

The ground connections heretofore referred to, it will be understood,may and in practice usually are one and the same, or

1 they may be the common otliee return, :l-

though it is found in practice that it is necesl sary to place a deadground upon one side of E the common battery. These ground connecl tionsmay be placed at the opposite pole ol l the several batteries instead ofas shown in the diagram, all of which is apparent to those l skilled inthe art. The pilot-relay may have I one and one-half ohms resistance andthe l other parts may be of the desired or usual di- 1 mensions.

figHaving thus described my invention, what I claim, and desire tosecure by Letters Patent, is-

1. The combination with a trunk-circuit extending between diflerentswitchboardsections and adapted to connect subscribers lines togetherfor conversation, of a trunkrelay at the incoming end legged to groundfrom the outgoing end of the trunk and actuated by current thereoverwhen connec tion exists at the outgoing end, a second relay at theincoming end of the trunk actuated by current over a portion of thetalking-circuit when connection is established with the called line, anda signal at the incoming end whose actuation depends upon said relays,and means ,for disconnecting said trunk-relay from the outgoing end ofthe trunk when the called subscriber answers, substantially asdescribed. 2. The combination with a trunk-circuit extending betweendifferent switchboard sections and adapted to connect subscribers linestogether for conversation, of a relay at the incoming end of the trunkconnected to ground from the outgoing end and actuated by current-overthe trunk when connection exists at the outgoing end but adapted to bedisconnected therefrom when the called sub-' scriber answers, a secondrelay at the incoming end actuated by current over a portion of thetalking-circuit when connection is es tablished with the called line,and a signal whose actuation depends upon said two relays, said signalbeing actuated when the connection exists at the outgoing end only togive a guard indication, and to act as a dis. connect-signal when thecalled, subscriber has hung up his receiver and connection has beensevered at the outgoing end, substantially as described.

3. The combination with a trunk-circuit extending between differentswitchboardsections and adapted to connect subscribers lines togetherfor conversation, of a disconnect-signal at the incoming end of thetrunk placed in condition to operate by current over a portion of thetalking-circuit when connection is established with the called line, anda trunk-relay connected between the outgoing end of the trunk and groundand actuated by current over the trunk when a connection exists at theoutgoing end, said relay serving when actuated to prevent the operationof said signal, and means to disconnect said relay from the outgoing endwhen the called subscriber responds, substantially as described.

4. The combination with a trunk-circuit extending between differentswitchboardsections and adapted to connect subscribers lines togetherfor conversation, of a disconnect-signal at the. incoming end placed incondition to operate by current over a portion of the talking-circuitwhen connection is established with a called line, a trunk-relay adaptedwhen energized to prevent the operation of said signal, said relay beingactuated by current over both sides of the trunk with suitable returnwhen connection is established at the outgoing end, and means fordisconnecting said relaywhen the called subscriber responds,substantially as described.

5. The combination with a trunk-circuit extending between differentswitchboard sections and adapted to connect subscribers lines togetherfor conversation, of a disconnect-signal for the incoming end of thetrunk, a relay actuated over a portion of the talking-circuit whenconnection is established with a called line to place said signal incondition to operate, a trunk-relay adapted when actuated to'prevent theoperation of said signal, said relay being connected at the incomingofiice between the outgoing end of the trunk and ground, a source ofcurrent likewise connected at the outgoing office when the connection isestablished at said end, a third relay responsive to current in the linewhen the called subscriber answers to sever the connection of saidtrunk-relay with the circuit, said third relay being arranged to preventthe operation of said signal while the trunk-relay is disconnected,substantially as described.

6. The combination with a trunk-circuit extending between differentswitchboardsections and adapted to connect telephone-. lines togetherfor conversation, of a disconnect-signal for the incoming end of thetrunk, a relay placed in condition to operate by current over a portionof the talking-circuit when connection is established with the calledline, a trunk-relay with its coil permanently connected to ground andadapted to be actuated over the two limbs of the outgoing end of thetrunk in parallel and ground, when a connection is established at theoutgoing end, said relay serving to prevent the operation of saidsignal, and a pilot-relay at the incoming end actuated simultaneouslywith said signal, substantially as described.

7. The combination with a trunk-circuit extending between differentswitchboardsections and adapted to connect telephonelines together forconversation, of a guardsignal atthe incoming end, a trunk-relayactuated over both sides of the circuit of the together forconversation, of a guard-signal for the incoming end of the trunk, atrunkrelay connected between both the outgoing ends of the trunk andground and actuated by current thereover when a connection existstherewith at the outgoing end to place said signal in condition tooperate, a second. relay at the incoming end actuated by current over aportion of the talking-circui t when connection is established with thecalled line and normally completing the circuit of said signal, and apilot-signal at the incoming end actuated simultaneously with saidguard-signal, substantially as described.

9. The combination with a trunk-circuit extending between d iflerentswitchboard-sections and adapted. to connect telephone-lines togetherfor conversation, of a cord-circuit at the outgoing end to connect withsaid trunk, a repeatingcoil having its windings connected across saidcord-circuit, a relay legged from the middle point of the windingsconnected with the trunk end of the cord-circuit to the live pole of abattery, a signal controlled by said relay, a trunk-relay at the i 11-coming end of the t 'unk legged to ground from the outgoing end, andmeans to sever the connection of said trunk-relay with the trunk whenthe called subscriber responds, whereby both the trunk-relay and thesupervisory relay at the outgoing end are rendered inoperative,substantially as described.

10. The COH'lbiiliLtiOll with a trunlecircuit extending betweenditlerent switchboard-sections and adapted to connect telephonedinestogether for conversation, of a disconnectsignal at the incoming end ofthe trunk, a source of current at said end, a relay also at said end toplace said signal in condition to operate and actuated by current fromsaid source over a portion of the talking-circuit when connection ismade with the called line, a trunk-relay at the incoming end connectedbetween the outgoing end of the trunk and ground and actuated by currentsent over the trunk from the outgoing end when connection is establishedat said end, said trunkrelay serving to prevent the operation of saidsignal when energized, a third relay at the inconnng end responsive tocurrent in the line when the called subscriber answers, said relayserving to disconnect the trunlerelay and to prevent the actuation otsaid signal when energized, said current source serving to charge thetransmitters t'or talking as well as tor the opera-thin of some of saidrelays, substantially as described.

11. The combination with a trunk-line extending between. diti'ercntswitcbboard-scctions, o'l' a cord-circuit adapted to be connected withsaid trunk at its outgoing end, and a *alled subscribcfs line with whichits inconiing end is adapted to be connected, a superyisory relayassociated with the cord-circuit and trunk-relay associated with theincoming end of the trunk-line, said relays being adapted to be actuatedSl1]l1llt:l1i(0ilSl by current over the outgoing end ot' the trunk,ground, and through their windings in series when connection isestablished at the outgoing end of the trunk, of signals associated, onewith each oi said relays and adapted to be displayed by the actuationl'liOlttYt, a second relay associated with the incoming end oi the trunkactuated when connection is es tablished with the c: lled line to renderone ot' said signals inoperative and a third relay associated w th thesame end ot the trunk actuated when the called subscr ber responds torender the other ot said signals inoperative, substantially asdescribed.

12. The combination with a trunk-line (X- tending between diil'crcntswitchboaribscctions, ot a eord-circuit adapted to be connected withsaid trunk at its outgoing end, and a called subscribcus line with whichsaid trunk is adapted to connect at its incoming end, of a supervisoryrelay associated with the cord-circui t, and a trunk-relay associatedwith the incoming end of the trunk-line,said relays being in series withthe two limbs ot' the trunk-line in par: llel, and having their circuitcontrolled by the position ot the called subscribens receiver-hook,substan ti ally as described.

Signed by me at Chicago, county ot' (Took. State of Illinois, this 20thday of July, 1W2.

FRANCIS W. DUNBAR.

Witnesses Ronnlrr Lnwls Anus, G AZELLE B EDER.

